Electrophotographic element having a series of alternate photoconductive and insulating layers

ABSTRACT

AN ELECTROPHOTOGRAPHIC ELEMENT IS MADE HAVING N PHOTOCONDUCTIVE INSULATING LAYERS AND N-1 INSULATING LAYERS RESPECTIVELY INTERPOSED BETWEEN SAID N PHOTOCONDUCTIVE INSULATING LAYERS, WHERE N=2,3 . . . I-1, I, . . . N. THE PHOTOLECTRIC CURRENT GENERATED IN THE   ITH PHOTOCONDUCTIVE LAYER IS GREATER THAN THAT GENERATED IN THE (I-1)TH PHOTOCONDUCTIVE LAYER, THE (I-1)TH LAYER BEING ON THE EXPOSED SIDE OF THE ELEMENT.

July 25, 1972 KATSUQ MAKINO ETAL 3,679,405

ELECTROPHOTOGRAPHIC ELEMENT HAVING A SERIES OF ALTERNATE PHOTOCONDUCTIVEAND INSULATING LAYERS Filed Aug. 26. 1968 2 Sheets-Sheet 1 l q; 2; 2 4 2FIG. IA FIG. I B FIG. IG

SURFACE POTENTIAL TIME AFTER EXPOSURE OPTICAL DENSITY INVENTORSLOGARITHMIC EXPOSURE ILLUMINATION KATSUO MAKINO IWAO SAWATO FIG. 3

July 25, 1972 KATSUQ MAKINQ EI'AL 3,679,405

ELECTROPHOTOGRAPHIC ELEMENT HAVING A SERIES OF ALTERNATE PHOTOCONDUCTIVEAND INSULATING LAYERS 2 Sheets-Sheet 2 Filed Aug. 26. 1968 0 TIME AFTEREXPOSURE FIG 6 LOGARITHMIC EXPOSURE QUANTITY I IflIII r Ewzma 2250 FIG]LOGARITHMIC EXPOSURE QUANTITY 1N VENT 0R5 KATSUO IIAKINO IWAO SAWATOEmzma EEEQ v I ATTORN 3Y5 1U nited States Patent ()fli 3,679,405Patented July 25, 1972 US. Cl. 96-1-5 14 Claims ABSTRACT OF THEDISCLOSURE An electrophotographic element is made having nphotoconductive insulating layers and n-1 insulating layers respectivelyinterposed between said n photoconductive insulating layers, where n=2,3 il, i, IN. The photoelectric current generated in the ithphotoconductive layer is greater than that generated in the (i'l)thphotoconductive layer, the (il)th layer being on the exposed side of theelement.

BACKGROUND OF THE INVENTION Field of invention The present inventionrelates to electrophotography, and more particularly to a photosensitivematerial used in electrophotography.

[Description of prior art Electrophotography is defined as a techniquewherein copies of photographs or prints are made by charging anelectrophotosensitive material and image exposing the photosensitivematerial to form an electrostatic latent image on the surface of aphotoconductive insulating layer. The electrostatic latent image isdeveloped with the electrostatic forces of line colored chargedparticles.

In electrophotography a photoconductive insulating material whichconsists of photoconductive fine powders containing mainly amorphousselenium or cadmium sulfide dispersed in electric insulating binder isused. Besides, zinc oxide, titanium oxide and the like is normally usedas the fine photoconductive powder. The photoconductive powder isdispersed in the electric insulating binder and coated on a conductivebase such as a metal plate, metal sheet, paper sheet or plastic sheettreated with conductive material to form an electrophotosensitivematerial. This material is uniformly charged on the photoconductiveinsulating layer by corona discharge or the like. The charge ismaintained according to its dielectricity in the dark portion of thephotoconductive insulating layer. When the photoconductive insulatinglayer is exposed with a picture image, the charged insulating layer isdischarged to an extent proportional to the exposed light intensity soas to form a charge pattern on the surface of the photoconductiveinsulating layer. The electrostatic latent image formed on the surfaceof the photoconductive insulating layer as described above is thendeveloped for instance by being cascaded with charged colored resinpowder. Since some of the charge remains on the unexposed areas of thephotoconductive insulating layer, a large amount of the colored resinpowder will adhere to the unexposed areas and a high density powderimage will be created on the photoconductive insulating layer. When thecolored resin powder is a thermoplastic, the powder image is fixed byheat fusing.

There is no established theory explaining how the static charge on thecharged photoconductive insulating layer is discharged by light exposurebut it is believed to occur as follows: Aetinic light is absorbed nearthe surface of the photoconductive layer. The free charge carriers carryphotoelectric current exclusively in the vicinity of the surface of thephotoconductive insulating layer. It is therefore necessary in order toincrease the sensitivity, to transfer these free charged carriers asclose as possible to the base, that is to lengthen the range of the freecharge carrier as much as possible. Alternatively, the sensitivity canbe indirectly increased by making the thickness of the photoconductiveinsulating layer as thin as possible, or by making it possible for theactinic light to penetrate the photoconductive insulating layer. In anyevent, it is most important that the range of the free charged carriersof the photoconductive insulating layer of high sensitivity be as longas possible.

The electrophotographic gradation of the electrophotographicphotosensitive material with the photosensitive layer of photoconductiveinsulating material having a long range free charged carrier, asdescribed above, is generally sharp. That is, the density varies fromfog density to maximum density in the range of 0.8 to 1.0 on a logexposure value scale graded. This sharpness is very favorable forcopying a line drawing or stipple but is unsuitable for copying picturephotographs having a continuous gradation.

SUMMARY OF THE \INVEN'TION The present invention provides anelectrophotographic photosensitive material having a gradualphotographic gradation using photoconductive insulating materials andhaving long range free charged carriers. The present invention furtherprovides a method of manufacturing the electrophotographicphotosensitive material which can change the gradation of the photographfrom sharp to gradual by use of a highly sensitive photoconductiveinsulating material having long range free charge carriers.

The electrophotographic photosensitive material provided by the presentinvention comprises a base and. a photosensitive layer. The base is notessential and may be omitted. The base may be conductive or not, and maybe transparent or opaque to light or radiation. The photosensitive layerconsists of at least two photoconductive insulating layers and a thininsulating layer sandwiched between said photoconductive insulatinglayers. These photoconductive insulating layers should be able to meetthe following conditions:

The unexposed side of the photoconductive insulating layer has a highersensitivity in the spectroscopic region than in the exposed side of thephotoconductive insulating layer. Any photoconductive insulating layerbetween the opposite sides of the photoconductive insulating layers hasa lower sensitivity than the unexposed side of the photoconductiveinsulating layer and higher sensitivities than the photoconductiveinsulating layer on the exposed side in the spectroscopic region or inanother spectroscopic region. Each photoconductive insulating layer istransparent to light within at least a portion of the spectroscopicrange where the unexposed side of the photoconductive insulating layersis sensitive.

The exposed side of the photoconductive layer is the photoconductiveinsulating layer, on the top of the photosenstitive material, formed byproviding a photosensitive layer on the opaque base, and the unexposedside photoconductive insulating layer is the photoconductive insulatinglayer at the bottom of said photosensitive layer. In both cases, thephotosensitive material is exposed from the photosensitive layer side.When the photosensitive material having the photosensitive layer isprovided on a transparent base and is exposed from the base side, thephotoconductive insulating layer on the top of the photosensitivematerial becomes the unexposed side of the photoconductive insulatinglayer and the photoconductive insulating layer on the bottom of thephotosensitive layer becomes the exposed side of the photoconductiveinsulating layer.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. l-a, l-b and 1-c arecross-sectional views of photosensitive material showing the structureof the conventional single layer and semisingle layer photosensitivematerial;

FIG. 2 is a diagram showing the variation with exposure time of theelectric surface potential of the electrophotosensitive material shownin FIGS. l-a, l-b and 1-0;

FIG. 3 is a diagram showing the relation between the log exposure valueand the optical density of the photosensitive layer consisting of asinglp photoconductive insulating layer;

FIG. 4 is a cross-sectional view of an embodiment of theelectrophotosensitive material in accordance with the present inventionconsisting of two photoconductive insulating layers;

FIGS. 5 and 6 are diagrams showing variations with exposure time of thesurface electric potential of the electrophotosensitive material shownin FIG. 4;

FIG. 7 is a diagram showing the characteristic curve representing thevariation of the optical density with log exposure value of thephotoconductive insulating layer consisting of two layers shown in FIG.4;

. FIG. 8 is a cross-sectional view of the electrophotosensitive materialconsisting of three photoconductive insulating layers; and

FIG. 9 is a diagram showing the similar characteristic curve, as oneshown in FIG. 7, of the electrophotosensitive material shown in FIG. 8.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The photosensitive layer,in accordance with the present invention, preferably is applied to aconductive base. Each photoconductive insulating layer has long rangefree charge carriers. Desirably, the range of the free charge carrier islonger than Lu. The free charge carries may be electrons or positiveholes and both may be characterized -by a long range. Eachphotoconductive insulating layer may be constructed with quite differenttypes of material so long as the layer meets certain necessaryconditions. For instance, a photosensitive material prepared by forminga layer of red sensitive photoconductive powder dispersed in aninsulating binder on a conductive base, an insulating thin layer isformed thereon, and a uniform layer of bluegreen sensitive amorphousselenium is deposited thereon. The photosensitive material is thenexposed from the selenium side. The purpose of the insulating layer isto prevent the free charge carriers generated in each photoconductiveinsulating layer from moving to the adjacent photoconductive insulatinglayer and drifting therein. Therefore the thin insulating layer shouldbe able to prevent the drift of the free charge carriers. If therequirements are met, the insulating layer is as thin as possible. It isnot advantageous for the insulatinglayer to be completely insulating.

Basically two types of photosensitive material are provided by thepresent invention; one in which the spectroscopic range of eachphotoconductive insulating layer is different from that of the others,and one in which each photoconductive insulating layer has the samespectroscopic range but a difierent sensitivity from the others. .Mixedtypes of the two are also contemplated within the scope of thisinvention.

Powders containing large proportions of cadmium sulfide or cadmiumcarbonate are used as a photoconductive powder. Superiorphotoconductivity is obtained with photoconductive powders havingcadmium iodide adsorbed thereon. The photoconductive powder isdye-sensitized with a dye and the sensitivity or spectral-sensitivitywithin non-coloring photoconductive insulating layer has a sensitivityof between 400 m and 600m The photoconductive powder is sensitized byvarious types of coloring matter in various spectral-sensitive regions.It is possible to increase the spectral-sensitive range by adding othermaterial other than a coloring material. For instance, thespectral-sensitive region can be broadened by substituting part of thesulfur in the cadmium sulfide with selenium. Variation of thespectral-sensitivity is not however a basic problem of this invention.

The photoconductive powder containing mostlycadmium sulfide and cadmiumcarbonate with adsorbed cadmium iodide, can be sensitized to. varioussensitivities by dye-sensitization. Another type of photosensitivematerial having a photosensitive layer is obtained by applying a layerdispersed with a highly sensitized photoconductive powder, applying alayer dispersed with a poorly sensitized photoconductive powder, andinterposing therebetween a thin insulating layer. The highly sensitizedpowder is exposed to light which is not completely absorbed into thepoorly sensitized photoconductive powder where the absorbed lightgenerates photoelectric current. Photoelectric current is also generatedin the unexposed layer. Since the actinic light intensity in theunexposed side is normally low, the desired sensitivity should be higherthan at the exposed side.

A photosensitive layer consisting of two photoconductive insulatinglayers has so far been described, but a photosensitive layer havingthree or more layers can be prepared in basically the same way.

The desired property of the photoconductive insulating layer orinsulating thin layer for making a complex photosensitive layer inaccordance with present invention is summarized as follows: i

(l) The range of the free positive holes and/or free electrons of thephotoconductive insulating layer is long.

(2) The thin insulating layer is able to prevent the free charge carriedgenerated in the photoconductive insulating layer from moving into ordrifting into a photoconductive insulating layer on the opposite side ofthe thin insulating layer.

(3) The photoconductive insulating layer and the insulating layer arelight transparent in at least a part of the spectral-sensitive region ofthe photoconductive layers at a position nearer to the unexposed sidethan the photoconductive insulating or the insulating thin layers.

(4) Each photoconductive insulating layer has a differentspectral-sensitive region from either that of others, or, when two ormore photoconductive insulating layers have the same spectralsensitivity region, the one which is closer to the unexposed side hasthe higher sensitivity in the spectral region.

FIGS. l-a, 1-b and 1-0 are cross-sectional views of photosensitivematerials showing the structure of the conventional single layer andsemisingle layer photosensitive material. In FIG. l-a a singlephotoconductive insulating layer is formed on a conductive base 1. Adescription of the invention will be set forth according to a case inwhich the photoconductive insulating layer consists of photoconductivepowder containing cadmium sulfide and cadmium carbonate with adsorbedcadmium iodide dispersed in a resinous binder. When the surface of thephotosensitive material is charged into negative polarity and exposed toblue light, the surface electric potential is attenuated as shown inFIG. 2 with the curve 1. The blue light generates a pair of freeelectrons and a positive hole in the vicinity of the surface of thephotoconductive insulating layer. As a result of free electron drifttoward base 1, the surface electric potential is attenuated as shown inFIG. 2 with the curve 1. If the photoconductive material is charged intoa positive polarity and exposed to blue light, the surface electricpotential is represented by the curve 2 in FIG. 2, which shows that itis dilficult for the positive holes to drift from close to the surfacetoward base 1. It is apparent that the photoconductive insulating layerhas a sufliciently long range of electrons and insuflicient range ofpositive holes. The thickness of the photoconductive insulating layeremployed here is about 80 u. The surface electric potential in case thatthe photosensitive material is charged negatively and exposed to redlight is shown in FIG. 2 with the curve 3, which shows that thephotosensitive material has little sensitivity for red. Thephotosensitive material shown in FIG. 1-b is made by applying thenon-sensitized photoconductive insulating layer 3 on a base 1 andapplying thereon a coloring matter sensitized photoconductive insulatinglayer 4. The surface electric potential of the photosensitive materialshown in FIG. l-b is represented by the curve 4 in FIG. 2 when chargedon the surface thereof into negative polarity and exposed to red lightfrom the photoconductive insulating layer 4 side. This shows that freecharge carriers are generated only in the photoconductive insulatinglayer 4 since the photoconductive insulating layer 3 is not sensitivefor red light, and that the generated free charge carriers move from thelayer 4 to the layer 3 and drift in layer 3 till the free chargecarriers reach the base 1 to attenuate the surface electric potential.That is, it is apparent from the above behavior of free charge carriersthat the carriers are able to move to the adjacent photoconductiveinsulating layer. The photosensitive material shown in FIG. l-c is madeby applying the coloring matter sensitized photoconductive insulatinglayer 5 on a base 1 and applying thereon a non-sensitizedphotoconductive insulating layer 6. The surface electric potential ofthe photosensitive material shown in FIG. l-c is represented by thecurve 5 and 6 in FIG. 2 when charged on the surface thereof intonegative polarity and exposed to red light. The curve 5 is one in whichthe thickness of the photoconductive insulating layers 5 and 6 is thesame, and the curve 6 is one in which the ratio of the thickness oflayer 5 to 6 is 1/ 9. In both cases, the free charge carriers aregenerated only in the coloring matter sensitized photoconductiveinsullating layer 5 since the exposure light is red light. The freepositive holes do not drift substantially, therefore the positive holesnever move into the layer 6 and drift therein. The free electrons driftonly in the layer 5 toward the base 1. Therefore, the surface potentialis attenuated to the extent corresponding to the moved amount of thefree charge carriers in the photoconductive insulating layer 5. Thesurface potential of this photosensitive material in the case that thesurface of the photosensitive material on the exposed side is chargedinto positive polarity and exposed to red light is represented by thecurve 7 in FIG. 2. In this case, the free electrons generated in thelayer 5 is moved into the layer 6 and drift up to the surface thereof.In the layer 5 on the other hand, even down in the layer near the base 1some amount of free electrons are generated and are moved toward thelayer 6. Therefore, the surface potential as a whole is attenuated asrepresented by the curve 7 in FIG. 2. As described hereinabove, in thecase that two photoconductive insulating layers are superposed in directcontact with each other without interposing any insulating layer, thefree charge carriers generated in one of the layers are easily movableto the other photoconductive insulating layer. Therfore even thephotosensitive layer consisting of two photoconductive insulating layersbehaves as if it consisted of single layer except as to thespectroscopic sensitivity. As described hereinabove and as shown in FIG.3, the semisingle layer and single layer have almost the samephotographic characteristic curve.

The ordinate in FIG. 3 represents the optical density when developedwith colored (black) charged resinous powder, and the abscissarepresents the log of the exposure light value. The part 8 of the curvein the figure represents the developed density of the image of unexposedor low exposure part. In the photosensitive layer described in the abovedescription, the density varies from the fog (almost zero) density tothe maximum density in the range of 0.8 to 1.0 in the log scale of theexpousure light value.

FIG. 4 shows an embodiment of the electrophotographic photosensitivematerial in accordance with the present invention. The dye-sensitizedphotoconductive insulating layer 9 is applied on a conductive base 8 andnon-sensitized photoconductive insulating layer 11 is formed thereoninterposing an insulating thin layer 10 to make an electrophotographicphotosensitive material. This photosensitive material is exposed fromthe photoconductive insulating layer 11 side. The photoconductiveinsulating layer 9 on the non-exposed side of this photosensitivematerial is panchromatic and the opposite photoconductive insulatinglayer 11 on the exposed side is sensitive to blue-green. That is, thisphotosensitive material has similar characteristics to that shown inFIG. l-c. But it is different therefrom in that an insulating layer isinterposed between the photoconductive insulating layers. When thisphotosensitive material is charged into negative polarity and exposed toblue-green light free electrons and positive holes are generated in thevicinity of the surface of the photoconductive insulating layer 11 andthe free electrons drift toward the base 8. But the free charge carriersare prevented from moving to the other layers by the insulating thinlayer 10. Therefore when exposed to blue-green light, only in thephotoconductive insulating layer 11 the charges are discharged, and thesurface potential is attentuated as represented by the curve 8 in FIG.5. When this photosensitive material is charged into negative polarityand exposed to red light, the surface potential is represented by thecurve 9 in FIG. 5. In this case, only in the photoconductive insulatinglayer 9 are the free charge carriers generated and only in this layerare the charges discharged. The curves in FIG. 5 represent the variationof the surface potential when the photoconductive insulating layers 9and 11 have almost the same thickness. If the thickness of the layersare different from each other, the residual surface potential afterexposure is different from that in said case wherein the thickness oftwo layers are the same. As apparent from the above description, in thephotosensitive material in accordance with the present invention shownin FIG. 4, the photoconductive insulating layers 9 and 11 are dischargedindependently, and the two layers never interfere with each other indischarging as the photosensitive material as shown in FIG. 1-c whereinthe photoconductive insulating layers 5 and 6 interfere each other.Accordingly, when the photosensitive material shown in FIG. 4 isnegatively charged, and exposed to red and then blue-green light or viceversa, the surface potential is attenuated as represented by the curve10 in FIG. 6. The part 12 of the curve 10 is a part attenuated in caseof exposed to red (or blue-green) light and the part 13 thereof is apart in case of exposed to blue-green (or red) light. The part 14 of thecurve 10 represents the potential when not exposed and disappears if thered (or blue-green) light then blue-green (or red) light are exposedsuccessively thereon, then the curve changes into the curve asrepresented by the curve 11 in FIG. 6. Since there is no failure of thereciprocity law in these photosensitive material, a curve of therelation of density to exposure as the curve 12 in FIG. 7 is obtained byexposing through a light intensity scale and developing the exposedphotosensitive material. The density varies from the fog (almost zero)density to the maximum density in the range of 1.6 to 2.0 on the logscale of the exposure value. This is almost twice as large as thevariable range of the photosensitive material consisting of singlephotoconductive insulating layer.

A photosensitive material having three photoconductive insulating layersis shown -with its cross-section in FIG. 8. This photosensitive materialis prepared by applying to a base 15 a photoconductive insulating layer16 highly sensitized for red light, and applying thereto aphotoconductive insulating layer 18 lowly sensitized for red light andinterposing an insulating thin layer therebetween. Further, onto thelayer is applied a non-sensitized photoconductiveinsulating layer 20 andinterposing an insulating thin layer 19. When this photosensitivematerial is negatively charged and exposed to blue-green light, the freecharge carriers are generated mainly in the photoconductive insulatinglayer 20 and only this layer 20 is discharged. When exposed to redlight, free charge carriers are generated in the photoconductiveinsulating layers 16 and 18, and the charges are dischargedindependently in respective photoconductive insulating layer, that is,the free charge carriers generated in the photoconductive insulatinglayer 18 do not move into the photoconductive insulating layer 16. Sincethe photoconductive insulating layers 16 and 18 are sensitive for redlight and the layer 16 on the unexposed side is sensitized higher thanthe layer 18 on the exposed side, even the attenuated red light reachingthe layer 16 through the layer 18, where it is absorbed to some extent,causes discharging in the layer 16 as much as in the layer 18. In thephotosensitive material shown in FIG. 8, the layers are independentlydischarged since the insulating thin layer is interposed between thephotoconductive insulating layers. Therefore in this case, as describedabove, the density varies from the fog density to the maximum density inthe range of the sum of the ranges of the exposure values on log scaleof the two photoconductive insulating layers when exposed through alight intensity scale and developed. This result is shown in FIG. 9.

As the three layers photoconductive insulating layer having a structureshown in FIG. 8, a photoconductive insulating layer consisting of threelayers having different spectroscopic sensitivity from one another, andas the photoconductive insulating layer consisting of two or threelayers having the structure as shown in FIGS. 4 or 8, a photoconductiveinsulating layer consisting of layers having the same spectroscopicsensitivity.

It should be readily understood that there may also be used aphotosensitive material having more than three layers as well as thephotosensitive material having two or three photoconductive insulatinglayers.

When the photosensitive material consisting of a plurality ofphotoconductive insulating layers is used, it is desirable that thecharacteristic curves are continued in series as shown in FIG. 9. Thisis controllable by regulating the spectral sensitivity, absolutesensitivity (e.g., the degree of dye sensitization), and thickness ofthe photoconductive insulating layers and the light property used.

The insulating thin layer prevents the free charge carriers generated ineach photoconductive insulating layer from drifting into the otherlayers as described above. If

the thickness of the insulating layer to the total thickness of thephotosensitive layeris too great, the non-sensitive terval of theelectrophotographic process such as charging, exposing, developing andthe like, and generally preferred to be 1 to 10 seconds. The purpose ofthe insulating thin layer is in eflfect to prevent the free positiveholes generated in the photoconductive insulating layer on the Iunexposed side from drifting and being captured and to prevent the freecharges generated in the photoconductive insulating layer on the exposedside from drifting and being captured or neutralize them. For thatreason, the term insulating thin layer may be changed to driftpreventing layer of the free charge carriers in a more strict sense. Inthis specification and in the appended claims, the term insulating thinlayer. is used and will be used as a term havingsuch meaning asdescribed hereinabove.

Therefore, the insulating thin layer is preferred to be extremely thinand to be a material having a low insulation resistance. The material ispreferred to a specific resistance of 10 -45 9 cm. and to have highresistance in the lateral direction.

Practically, various types of thin layer forming high molecularmaterials can be used, and a compound containing pigment powders whichare transparent for some light added to the high molecular material canalso be used as the insulating thin layer. The pigment powder may itselfhave some degree of photoconductivity. It is necessary, however, thatthese materials should have a short range free charge carrier, that is,the range of the free charge carries should not be longer than thethickness of the think layer. The thickness of the insulating thin layeris preferred to be less than 1,u.

Now the method of preparing and using the photographic photosensitivematerial in accordance with the present invention will be described indetail as to some examples thereof.

EXAMPLE 1 An electrophotosensitive material consisting of twophotoconductive insulating layers one of which is red sensitive on theunexposed side and the other of which is bluegreen sensitive wasprepared as follows:

Fine photoconductive particles were obtained by preparing a slurry whichwas made by adding 160 parts by weight of yellow-orange pigmentcadmium-yellow-orange #4700 (made by Mitsubishi Metal Co., Ltd.) and 40parts by weight of cadmium iodide into ethanol allowing it to ticles ofsensitized photosensitive material were obtained by adding to parts byweight of photoconductive fine particles. 0.1 weight part of MalachiteGreen, a sensitizing dye, dissolved in ethanol to make a slurry. Theslurry was allowed to stand for 24 hours and the ethanol was removed byevaporation at 80 C. Using this powder, a photosensitive coatingmaterial B was obtained by the same process as employed in preparing thenon-sensitized powder. The photosensitive coating material B was appliedon an aluminum plate which had been treated to remove grease by spraycoating. The Magicrone #200 clear was applied thereon as the insulatingthin layer, and the photosensitive coating material A was applied on theexposed side as the photoconductive insulating layer. After the aluminumplate with the layers thus coated was sufficiently dried at 70 C., theplate was heated for 30 minutes at C., whereby an electrophotosensitivematerial consisting of two photoconductive insulating layers havingstrong coating layers was obtained. The thickness of each layer afterheating was 20p. at the base side, 15p. at the exposed side, and about1p. at the thin insulating layer.

The photosensitive material consisting of two photoconductive insulatinglayers obtained as described above has a blue-green and red lightsensitivity. It was sensitive for the green light of a tungsten lamplight at 2700 K. through the Fuji Filter K# 17, and for the red light ofthe same light source through the Fuji Filter K#7. When exposed to amixed light of these two frequencies through an optical density wedgefor 0.5 second, and developed with a magnetic brush, this photosensitivematerial was developed with an image having a variable density from fogto the maximum density in a range on a log scale of the exposure oftwice as large as that of the photosensitive material consisting of asingle photoconductive insulating layer. That is, the photographicgradation is half of that of the photosensitive material consisting of asingle layer.

When preparing the photosensitive material in accordance with thisembodiment, a photosensitive material was made without applying theintermediate insulating thin layer. This photosensitive material wasalso sensitive to both blue-green and red light, but the photographicgradation was almost the same as that of the photosensitive materialconsisting of a single photoconductive insulating layer.

EXAMPLE 2 An electrophotographic photosensitive material consisting oftwo photoconductive insulating layers which is panchromatic at theopposite surfaces thereof and wherein the photoconductive insulatinglayer on the unexposed side has a higher sensitivity than thephotoconductive insulating layer on the exposed side, was prepared asfollows:

Dried sensitized photoconductive fine particles were prepared by addingan ethanol solution with a resolved sensitizing brilliant green coloringmatter of 0.1 part by weight to 100 parts by weight of non-sensitizedphotoconductive powder which was made by adding cadmium iodide tocadmium yellow orange #4700 obtained in the Example 1. The compositionwas formed into a slurry, left to stand for 24 hours and heated to 80 C.to vaporize the ethanol. A photosensitive coating material C wasprepared by adding 50 parts by weight of said Magicrone #200 clear to100 parts by weight of said photoconductive powder, and dispersing itfor 18 hours. By the same process, sensitized photoconductive powderwhich was made by adding 0.02 part by weight of sensitized coloringmatter brilliant green to 100 parts by weight of said nonsensitizedphotoconductive powder was prepared. Using the photoconductive powderphotosensitive coating, material D was prepared.

On a degreased aluminum sheet, said photosensitive coating material wasapplied, and another coating material made 'by the following process wascoated thereon to form an insulating thin layer. That is, a coatingmaterial made by adding 80 parts by weight of cadmium carbonate powderof less than 0.1a diameter to 50 parts by weight of said Magicrone #200clear. The composition as formed was dispersed for 24 hours in a ballmill and was thereafter used as the insulating thin layer. Onto thislayer, a photosensitive coating material -D was coated. The aluminumsheet with these layers coated was dried by heating for 45 minutes at 70C. and heated for 30 minutes at 150 C. (Thickness after heating was15 1. at the low sensitized layer, 20 at high sensitized layer, and I atthe insulating thin layer.)

The electrophotosensitive material obtained by the process as describedabove had a strongly sensitized layer of panchromatic. That is, thelayer was sensitive in the range of 400 m to 700 m of wavelength,although the sensitivity became somewhat attenuated around 550 m Theelectrophotosensitive material thus obtained was charged into a negativepolarity by corona discharge, exposed to white light through an opticaldensity wedge, and developed by magnetic brush methods. Thusphotographic gradation of about half of that of the photosensitivematerial consisting of single layer was obtained.

But when the photosensitive material was removed of its thin insulatinglayer, prepared at the same time, photographic gradation wasapproximately the same as that of the photosensitive material consistingof single layer.

EXAMPLE 3 Onto a conductive glass, the photosensitive coating materialA, prepared in Example 1, was coated with the thickness of 15 afterheating. A coating material containing Aerozyl (silica powder made byDegussa, West Germany) instead of the coating material containingcadmium carbonate, as in Example 2, was applied as the insulating thinlayer thereon. The thickness of the insulating thin layer after heatingwas less than 1.0 Onto this layer, the photosensitive coating materialD, as prepared in Example 2, was coated with a thickness of 15a. Saidcoating material for the insulating thin layer was applied at athickness of less than 111., and the photosensitive material C inExample 2 was applied thereon at a thickness of 30].! And this was driedfor 3 hours at 70 C. and heated for 30 minutes at C.

The electrophotographic photosensitive material prepared as describedabove was charged by corona discharge exposed from the conductive glass,that is, base side, and developed from the opposite side by a magneticbrush. The photographic gradation of the developed image was about /3 ofthat of the electrophotosensitive material consisting of a single layer.

The invention has been described in detail with reference to someembodiments thereof, but it will be understood that variations andmodifications can be effected within the spirit and scope of theinvention as described hereinabove and as defined in the appendedclaims.

What is claimed is:

1. An electrophotographic element comprising n photoconductiveinsulating layers where n=2, 3, i-l, i, N the first layer being on theexposed side of the element and the nth being on the unexposed side andn1 thin, continuous insulating layers respectively interposed andnon-removably connected between said n photoconductive insulatinglayers, the thickness of each said thin insulating layer being less than1 micron where the range of any free charge carriers generated in eitherof the photoconductive insulating layers on the respective oppositesides of said thin insulating layer is less than the thickness of saidthin insulating layer and the specific resistance of each of said thininsulating layers being in the range from 10 to 10 ohm-centimeters, andwherein the photoelectric current generated by radiation employed duringthe exposure step of the imaging cycle in the ith photoconductiveinsulating layer is greater than that generated in the (il)thphotoconductive insulating layer and where the (il)th photoconductiveinsulating layer and insulating layer between the (il)th and ithphotoconductive insulating layers transmit light in at least a portionof the spectral wavelength region to which the ith photoconductiveinsulating layer is sensitive.

2. An element as in claim 1 wherein n=2.

3. An element as in claim 2 where the second photoconductive insulatinglayer is sensitive to light over a [first wavelength range and wheresaid first photoconductive insulating layer is sensitive to light over asecond wavelength range smaller than said first wavelength range.

4. An element as in claim 3 where said first wavelentgh rangeapproximately extends from 400 to 700 millimicrons.

5. An element as in claim 4 where said second wavelength rangedapproximately extended from 400 to 600 millimicrons.

6. An element as in claim 2 where the second photoconductive insulatingelement is responsive to light over a first wavelength range and thefirst photoconductive layer is responsive to light over approximatelythe same first wavelength range, said first photoconductive insulatinglayer being less sensitive to light over said first waver 11 lengthrange than said second photoconducti-ve insulating layer.

7. An element as in claim 6 where said first Wavelength rangeapproximately extends from 400 to 700 millimicrons and said portionthereof approximately extends from' 600 to 700 millimicrons.

8. An element as in claim 1 where n-=3.

9. An element as in claim 8 where the third photoconductive insulatinglayer is sensitive to light over a first wavelength range and the secondphotoconductive insulating layer is also sensitive to light over saidfirst wavelength range, said second photoconductive insulating layerbeing less sensitive to light over said first wavelength range than saidthird photoconductive insulating layer.

10. An element as in claim 9 where said portion of said first wavelengthrange approximately extends from 600 to 700 millimicrons.

11. An element as in claim 9 where the first photoconductive insulatinglayer is sensitive to light over a second wavelength range smaller thansaid first wavelength range.

12. An element as in claim 11 where said first wavelength rangeapproximately extends from 400 to 7 00 millimicrons, said portion ofsaid first wavelength range approximately extends from 600 to 700millimicrons, said second wavelength range extends from 400 to 600millimicrons.

13. An element as in claim 1 where said thininsulating layer is selectedfrom the group consisting of acrylic resin, cadmium carbon-ate andacrylic resin, and silica and acrylic resin.

14. An element as in claim 13 where said photoconductive insulatinglayers are composed of inorganic materials.

References Cited UNITED STATES PATENTS 2,476,800 7/ 1949 Blackburn 96l.5X 2,962,374 5/1956 Dessauer 961.2 X 2,962,375 5/1956 Schaffert 96 1.2 X2,803,541 8/1957 Paris 961.5 X 2,901,348 8/1959 Dassaner et al. 96--1.5X 2,962,376 11/1960 Schaffert 11720l X 3,170,790 2/1965 Clark 961.53,312,548 4/1967 Straughan l17201 X 3,394,001 7/ 1968 Makino 96-1.53,468,660 9/1969 Davenport et al. 96-1.5 3,508,918 4/1970 Levy 961.5 V

GEORGE F. LESMES, Primary Examiner I. R. MILLER, Assistant Examiner U.S.Cl. X.R. 96-1 .6

